The ethylenically unsaturated compounds which can be polymerized by free radical initiation are commonly called monomers. They constitute a major class of industrial chemicals. Because of the presence of the polymerizable double bond, the widespread sources of initiating radicals from peroxides, light and/or thermal generation, such monomers are prone to undesirable and premature polymerization at various stages during their manufacture, purification, storage, shipping, blending and use. Protection of such monomers from such premature polymerization is needed up to the point where polymerization is actually desired. If premature polymerization does occur, the monomer may suffer contamination by polymer, troublesome increase in viscosity, gelation and/or loss of reactivity. Fouling of distillation equipment including heat exchanger surfaces, storage vessels, transfer lines, pumps, shipping containers and application equipment can occur with ensuing costs of cleaning, downtime, loss of material and unnecessary labor costs. A particularly difficult situation is the preparation of polyol acrylates from polyols and acrylic acid since prolonged heating periods are required to complete the esterification. Premature polymerization can also constitute a safety hazard since uncontrolled exothermic polymerization can cause ruptured vessels, atmospheric contamination, and in extreme cases, explosions and fires. Deterioration of monomers in shipping and storage may also make necessary the use of costly refrigerated shipping and storage facilities.
A further problem is that of undesired polymerization of adventitious monomers, that is, radically-polymerizable unsaturated monomers which occur in commercial products such as hydrocarbon fuels and refinery streams. In these cases, polymerization accompanied by the incorporation of oxygen moieties leads to gum and sludge deposits which can foul carburators, engines, fuel tanks or fuel lines. In refineries, the adventitious monomers in hydrocarbon streams such as cracking products can foul pipelines, valves, pumps, heat exchangers, stills and storage vessels.
Another problem in regard to undesired polymerization of free radical polymerizable monomers is the case of polymerizations which are intentional, but which must be prevented from going too far. For example, the quality of poly(vinyl chloride) suspension polymer and of synthetic rubber made from olefins and dienes is superior (i.e. better molecular weight distribution, stability, and processing properties) if the polymerization is stopped short of complete consumption of the monomers. It is also desirable to have available in a plant conducting vinyl polymerization reactions some rapid and efficient means for stopping a runaway polymerization if other means such as cooling should fail.
It is known that the addition of certain compounds to monomers can retard or even prevent their undesired polymerization, and that when polymerization of the monomer is desired, the inhibitor can be removed or overridden by a deliberately-added polymerization initiator. Various aromatic compounds have been used as such inhibitors in the prior art. Typical ones are hydroquinone, monomethyl ether of hydroquinone (MEHQ), tert-butylphenols, phenothiazine, phenylenediamines and benzoquinones. These are usually used at a level of 50 to 1000 ppm. These inhibitors are not totally effective and even with such inhibitors present, it is often advisable to store such inhibited monomers in a cool place and for limited periods of time. Moreover, these aromatic inhibitors are a cause of serious discoloration problems in the monomers and in polymers deliberately prepared from such monomers. Typically these aromatic inhibitors produce quinoidal chromophoric groups with very high visible light absorbance. The use of stable nitroxyl radicals as inhibitors also leads to discoloration since such compounds are themselves highly colored, usually bright red.
In order to overcome these color problems, a diligent search was made to find alternative inhibitors which are both effective and not discoloring. This search led to the N,N-dialkylhydroxylamines and the N,N-diaralkylhydroxylamines. Some typical references are cited infra.
U.S. Pat. Nos. 3,222,334 and 3,878,181 disclose the use of N,N-dialkylhydroxylamines such as N,N-diethylhydroxylamine as short-stopping agents for emulsion polymerizations of butadiene/styrene rubber and chloroprene.
U.S. Pat. Nos. 3,148,225 and 3,697,470 disclose the use of N,N-dialkylhydroxylamines such as N,N-diethylhydroxylamine and N-alkyl-N-arylhydroxylamine such as N-ethyl-N-phenylhydroxylamine respectively as short-stopping agents and popcorn polymer inhibitors in processes for preparing synthetic rubber. The popcorn polymer formation is a serious problem encountered in recovering of monomers from such synthetic rubber operations.
U.S. Pat. No. 4,782,105 teaches the use of long chain N,N-dialkylhydroxylamines as stabilizers to prevent the premature gelation of unsaturated elastomer compositions such as styrene/butadiene copolymers or polybutadiene.
U.S. Pat. No. 3,408,422 describes the use of N,N-dialkylhydroxylamines such as N,N-diethylhydroxylamine and N,N-diaralkylhydroxylamines such as N,N-dibenzylhydroxylamine as stabilzers for preventing the premature gelation of unsaturated polyesters.
U.S. Pat. No. 4,798,889 teaches the use of N,N-dialkylhydroxylamines such as N,N-diethylhydroxylamine or N,N-dibenzylhydroxylamine as stabilizers to reduce the thermal polymerization of organosiloxanes substituted by ethylenically unsaturated moieties.
U.S. Pat. Nos. 4,409,408 and 4,434,307 disclose the use of N,N-dibenzylhydroxylamine in combination with an alkylated diphenol (catechol or hydroquinone) as inhibitors to prevent the polymerization of styrene.
The use of stable nitroxyl radicals including those derived from hindered amine moieties has also been disclosed. Typical references are cited below.
Russian Published Application No. 1,139,722 describes the inhibition of styrene and comonomers such as butadiene using 1-oxyl derivatives of hindered amine compounds such as N,N'-bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)adipamide. The elimination of popcorn polymer and of the clogging of equipment is touted as the result of using such 1-oxyl compounds.
Japanese Sho 60-36501 describes the use of hindered amines and their 1-oxyl and 1-alkyl derivatives as vinyl polymerization inhibitors to improve storage stability of monomers such as acrylate and methacrylate esters.
European Patent Application No. 178,168 and British Patent No. 1,127,127 describe the use of 1-oxyl substituted hindered amine compounds as stabilizers for inhibiting the polymerization of .alpha.,.beta.-ethylenically unsaturated monocarboxylic acids, such as acrylic acid, during its recovery by distillation.
U.S. Pat. No. 4,670,131 teaches the use of 1-oxyl substituted hindered amine compounds as stabilizers for preventing the fouling of equipment for processing organic feed streams containing olefins by inhibiting the polymerization of said olefins.
In a theoretical study of the inhibiting effects of selected hindered amine compounds, Y. Miura et al., Makromol. Chem. 160, 243 (1972) disclose that 1-oxyl-2,2,6,6-tetramethylpiperidin-4-one is highly effective in retarding the onset of the polymerization of styrene and methyl methacrylate. By contrast, the corresponding 1-hydroxy-2,2,6,6-tetramethylpiperidin-4-one is stated to have no effect in delaying the polymerization of styrene and only a weak retarding effect on said polymerization once begun.
U.S. Pat. Nos. 4,668,721 and 4,691,015 disclose the use of 1-hydroxy substituted hindered amine compounds as stabilizers for polyolefin compositions in combination with one or more other stabilizers such as phenolic antioxidants, ultraviolet light absorbers and the like.
None of these references describes or suggests that 1-hydroxy substituted hindered amine compounds are or could possibly be effective inhibitors to prevent the premature polymerization of monomers in either the liquid or vapor phase.